If you know something about romanization and want to add here rules you are welcome, I will add them to the convention.
These will be useful for any newcomer that wants to collaborate.
Not only rules are welcome, but hints and common errors too. Maybe a couple of examples with boxart and logo...

Kazumi and hkA did an excellent work with suggestions and edits, but I can't see standard written rules. My poor contribution to this can only be the dump of the DOM FORUM's discussions about Japanese naming and romanization.
You can find the dump here: http://datomatic.no-intro.org/romanization-j.html

Maybe something is not related. Feel free to report me any thread (first number on the left) to be fixed or removed.

Thank you for compiling those documents. I have been thinking of these rules too.
From the forum posts and the 2007 Naming Convention, I have pulled out the following:

1. Baseline is using Hepburn convention, and the version of Hepburn that we seem to use (modern) recognizes
へ = e not he
を = wo not o

+ All loan words are rendered as their English equivalents instead of transliterated
e.g. アイテム (aitemu) = Item, ゲーム (geemu) = Game
- But Japanese-specific compounds are left as is like Pachi-Slot

+ Exceptions for common Japanese words with non-Hepburn romanizations
Tokyo, Kyoto, Osaka, there must be more
- maybe there are examples of games that have non-Hepburn or other unusual romanizations used overseas? I can't think of any but there must be some...
e.g. is it called Ryu ga Gotoku anywhere instead of Ryuu ga Gotoku?

+ Capitalization:
- Particles (no, wa, ga, e, ni, wo, ka, de) are uncapitalized unless at the beginning of a title or subtitle (and separated out by spaces)
- but does this extend to compounds like では (dewa) でも (demo) じゃ (ja) and forms of to be like ない (nai) and だ (da)? I think Nai (as in ja Nai, dewa Nai not as a verb ending Hatarakanai of course) and Da should be capital even though they are short because in English we do the same with Be and Is. dewa and demo I would put together (i.e. not de wa and de mo) but lowercase. My suggestion.
ex. Boku no

some decisions to make on common suffixes, as the usage is different in different sets/contexts right now:

In general it seems suffixes are hyphenated and lower case, like the usual name ones -san, -kun, -chan, -sama, but there are more to consider

版 (ban) e.g. 完全版 (kanzenban, complete version) 決定版 (ketteiban) kind of "version": do we keep it with nospace compounds even if it is not a traditional word, Kanzenban, Kanzen-ban, or Kanzen Ban? I guess I vote for nospace if it's a common one like kanzenban, or hyphenated otherwise? But that's maybe not good enough for a standard.
ex. Moe Moe Daisensou - Gendaiban+ (Japan) (v1.01)
Nazotte Oboeru - Otona no Kanji Renshuu - Kanzen Ban (Japan)
Dasshutsu Adventure - Kamioroshi no Uranai-ban (Japan) (eShop)

And lastly for me working on digital games with thousands of DLC, I get a lot of titles with things like 追加 (tsuika) ("additional") e.g. 追加アイテム 追加レベル Tsuika Item, Tsuika Level
We always transliterate game titles and subtitles and don't translate since that is subjective, so I will try to do the same for DLC titles, though it does end up with a lot of DLC titles like.
Game Japanese Title - Tsuika Item - Bushi no Kabuto (Japan) (DLC)

whereas the English game might have the DLC Game English Title - Additional Item - Warrior's Helmet, so USA and Europe will be matched up in alphabetical order, but it would be misleading to name the DLC the same way if it isn't listed like that on any store.

Also tangentially related since they are used in Japanese game titles a lot:
Game Title ~SUBTITLE~ becomes Game Name - Subtitle (Japan)
Game★Title or similar becomes Game-Title (Japan) not Game Title (Japan)?
Chaos;Curse is left as is
Re:Game Title becomes Re-Game Title (Japan) not Re - Game Title (Japan)?

Also, the question is how to choose the on/kun variants for Kanji. And there would be still many 'narrow', hard-to-describe cases, like 川 is "-kawa" for Japanese rivers and "-gawa" for the foreign rivers.

norkmetnoil577 wrote:+ All loan words are rendered as their English equivalents instead of transliterated

But don't forget about loaned and "mutated" words, like, "Torokko" was mutated from "Truck", but is a Japanese word now.

norkmetnoil577 wrote:- But Japanese-specific compounds are left as is like Pachi-Slot

Are you sure about Pachi-Slot? Not Pachislo? Also, Prowres, ProWres, Pro Wres or Pro-Wres? What's about shortened terms in general? Famires/FamiRes (shortened from Family Restaraunt), BuilCrush (shortened from Building Crush), etc.

What do you think about prefixes and how to determine, whether to write it as a prefix or as a compound word? Like, "Dai-" is a prefix, but "Daisenryaku" is a compound word?
Should the words like "Den" be hyphenated? Like, Shin Shinobiden or Shin Shinobi-den? Or Shin Shinobi Den? Toushinden, Toushin-den or Tou Shin Den?

norkmetnoil577 wrote:+ Exceptions for common Japanese words with non-Hepburn romanizations
Tokyo, Kyoto, Osaka, there must be more
- maybe there are examples of games that have non-Hepburn or other unusual romanizations used overseas? I can't think of any but there must be some...
e.g. is it called Ryu ga Gotoku anywhere instead of Ryuu ga Gotoku?

What do you think about names? Like, Samurai Spirits: Zankurou Musouken or Samurai Spirits: Zankuro Musouken?
What do you thing about company names? Whether to leave "Famitsuu" or to change to "Famitsu"? (certain games exist as Famtsuu-ban/Famitsu-ban)

norkmetnoil577 wrote:Also tangentially related since they are used in Japanese game titles a lot:
Game★Title or similar becomes Game-Title (Japan) not Game Title (Japan)?

★ (as well as ♥ and some other symbols) is a replacement for ·
Game·Title can be Game Title, Game-Title, Game & Title, Game, Title, but how to find out which case should be used?

What's about the corner brackets (「 」)? Should be replaced with ' symbols or omitted?
What's about pre-titles?
What's about genre names, that often go before the title? Japanese publishers seem to like those very much.
What's about the translated or romanized titles that often included under the main title? Should they be added as subtitles? Or added as the main titles ignoring the romanization rules? Or omitted?

Also, there are cases, when the Kanji symbols are transcribed with kana differently, like Mahou Knishi Rayearth is transcribed as Magic Knight Rayearth and there's no problems with that, the kana transcription is used. But what to do, when we have some acronym, that is also transcribed with kana? How to write it? GS (Ghost Sweeper) Mikami? GS Mikami? Ghost Sweeper Mikami? Goal FH? Goal Field Hunter? Goal FH (Field Hunter)?

My examples are mostly from the CD consoles world, but the ROM-based ones surely need the answers as well